Expensive underpowered PPC boards that are approved to run it, you can't run it on a cheap used PPC Mac. These are essentially lite versions of a G3 running at 756 MHz, think mobile phone. OS4 is light on resources so it runs fine on this kind of equipment.
Just to refine:
- to get most recent (4.1) AmigaOS working,
- - the only hardware in production is Sam, costing 400-500 Eur as a motherboard, 700-800 MHz, low spec in general
- - other hardware is used, expensive and practically impossible-to-get Pegasos 2, AmigaOne, microA1
- to get most recent (2.3/2.4 coming) MorphOS working,
- - key-performance and cheap hardware is Mac Mini PPC, the said 2.4 version is to be released shortly
- - other used hardware difficult to get is Pegasos 2 (Peg1 not sure if supports 2.3)
- - also the discontinued Efika PPC, low-specs but cheap, hard to get
To summarize - best option for AmigaOS4.1 is Sam (there are variants), for MorphOS2.4 is Mac Mini PPC (various specs as well), but this version is announced yet not available.